File-level backup applications copy data from the source volume to backup server on a file-by-file basis, regardless of where the data for the file is physically located on the original volume. This means that the backup application attempts to build and replicate the original volume's file data by reading the logical file system present on it. Block based backup applications, on the other hand, bypass the file system and read volume data directly from the disk or volume. The advantage of bypassing the file system is that there is no penalty on backup performance if the underlying disk or volume has large number of files. Block level backups implicitly preserve the file system layout of the original source volume in the backed-up image, which eliminates the need to generate and manage file indexes for granular recovery.
Disaster recovery from image level backup typically involves mounting the backup on the host where disaster recovery has to be performed and then reading the mounted volume's logical device object. The data is then sequentially written on to the raw target physical disk. However, if the backup image is stored in a format that is not directly mountable, recovering all of the image data is neither simple nor efficient.